| Summary: | Regional patterns of wintertime sea level pressure (SLP) trends over the North Pacific and their uncertainty were investigated based on the phase 3 of the Coupled Model Intercomparison Project (CMIP3) multi-model pro-jections under the Special Report on Emissions Scenarios (SRES) A1B emission scenario for the 21st century (2000–2099). While the 24-model ensemble mean of the 100-yr SLP trend over the North Pacific shows a north-ward shift of the Aleutian low (AL), regional patterns of the SLP change vary among the models. Projected changes deepen the AL in several models but it shifts northward in some others. The di¤erent response of the AL results in a large inter-model spread over the North Pacific, which is largest of the Northern Hemisphere and comparable in magnitude to the ensemble mean in the same region. This large spread means a high degree of uncertainty in the 100-yr SLP trend over the North Pacific. For the total uncertainty in the SLP trends over the North Pacific, we examined the relative importance of the internal climate variability and model uncertainty due to di¤erent treatments of physical processes and computa-tional scheme. To evaluate each of contributions, a single-realization ensemble using a subset of 10 CMIP3 mod-els is compared to a multi-realization ensemble for the same models in the A1B projections. Additionally the con-trol simulations under preindustrial conditions are examined to evaluate the background internal variability in
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